This invention relates to can ends and more particularly to the bottom end of a two-piece drawn steel food container which is designed to be packed and then processed at high temperature and pressure in a retort. More particularly, in a can where the contents are to be heated above their boiling point and then cooled, the bottom end is subjected to internal pressure and then external pressure. Each sealed container must be retorted to prevent bacterial growth and spoilage which will generate metabolic products such as organic acids and carbon dioxide; the latter tending to inflate the sealed container causing it to bulge or become unseamed. In order to have commercial sterility (safety) the food must be heated to a state which renders it free of viable forms of micro-organisms which are there or which would reproduce in the future under normal storage conditions. A certain group of high acid foods need not be retort processed, but these acidic foods are hot packed. That is to say that, they are heated to near the boiling point and then packed in the container. Even hot packing places considerable stress on the container. The combination of the hot fill and the acid is equivalent to retort sterilization of low acid level foods. Foods with a pH level of 4.6 or higher must be retorted in order to achieve commercial sterility.
In the past it has been the practice to use heavy gauge high strength metal to resist the processing stresses in a double seamed on bottom end for a three-piece container e.g. 85 lbs per base box plate. In general, the three-piece container is less satisfactory because it costs more, it may require soldering, it produces a needlessly heavy container, it is subject to side seam and double seam leaks and it is wasteful of energy and resources. More process steps are necessary during the fabrication of a three-piece container. More particularly, such containers include on their ends a deep chuck countersink for strength and chuck clearance, and such a countersink is subject to buckling during processing.
A two-piece can with an integral bottom does not require a bottom end chuck countersink for double seaming, but a bottom recess is necessary in order to manufacture a two-piece can with the same height and capacity as a conventional three-piece can so that either may be interchangeably used in the same packing and processing line. Profiling has been used to apply ribs, creases and the like to add rigidity to the bottom of a two-piece can. The weakest area of a drawn two-piece can is the bottom and consequently, the material thickness of the steel is a function of the bottom strength required (buckle resistance). With only moderate profiling, the pressurized two-piece can bottom may tend to distend and exceed the elastic limit of the metal. When that happens the can is unacceptable as it will rock about its distended bottom and appear to contain tainted or spoiled contents.
Consequently, a bottom recess can improve the performance of thin-two-piece cans if such a recess is designed to include work hardened areas of metal which increase the elastic limit of the bottom metal particularly in areas of high stress. The highly worked bottom is more rigid. Spoiled contents will tend to force outward the top end and simplify the process of checking the packed cans. The present invention achieves the desired rigidity and especially internal pressure resistance at the bottom of a two-piece can as a result of specific and cooperative profiling relationships wherein further appropriate work-hardening prevents highly undesirable eversion of an inwardly domed bottommost configuration.
The large capital investments in equipment for handling three-piece cans in the packing plant cannot be merely written off. A two-piece container which will physically resemble the three-piece container is essential in order to permit continued use of the existing three-piece equipment, e.g., labelling, runways, retort, etc. The 100% interchangeability is recognized in the patent art, see, for example, U.S. Pat. No. 3,912,109 which discloses an approach and several methods of making same. Similarly, see U.S. Pat. No. 3,272,383 which discloses an extruded aluminum can with a recessed bottom profile. Such prior art is typical in its emphasis on duplicating the shape of the three-piece can but fails to teach a container which is of adequate strength and will protect food without corrosion and move through customer's equipment. For economy and high-speed production a drawn two-piece can made of coil coated metal or subsequently coating a drawn and beaded container should be at the bottom of the can. It is very difficult to form a chime-shaped bead without damaging the inside coating surface. More particularly, the use of internal tools to form a chime-shaped bead is detrimental to the coating. In the present invention, the forming of a simple bead of prescribed shape is moderate by comparison and prevents undue loading by the inside tools necessary to form a chime-shaped bead. Consequently, the danger of scuffing and sharp bends in reforming which would crack the inside coating is completely eliminated because there is sufficient space inside the bottom can corner to operate a beading tool.
Since drawn two-piece containers offer numerous advantages particularly in the elimination of the side seam and one end seam, it is commercially important that the bottom profile formed in accordance with the preferred method and shape also be able to withstand a retort temperature of 260.degree. F. for thirty minutes or more, and yet be interchangeable in all respects with the three-piece container which from time to time may have to be used on the same package and processing designed so that the interior coating remains intact even though the bottom is deeply recessed and formed to include a chime-like bead to provide rolling in the trackwork and through the labeler and other food packing and processing equipment. The prior art teachings show chime-shaped beads.
It is, therefore, an object of the present invention to provide a two-piece can bottom profile which is resistant to ultimate stress by improving the elastic limit of the can material particularly in the bottom.
It is yet another object of the invention to provide a sidewall bead, located just above the corner between the bottom and wall, that will have a prescribed shape and an overall outer diameter essentially equal to outer diameter of the top double seam of the closed container.
It is still another object of the present invention to provide a bottom profile and lower sidewall bead which will allow two-piece cans to be run interchangeably with conventional three-piece cans, such that the processing speed of the two types of containers are essentially the same.
It is an object of the invention to provide a bottom profile and method for forming same which will not destroy the integrity of the coated interior.
It is a further object of this invention to provide a two-piece container of a low cost efficient light gauge coated metal which is capable of resisting buckling caused by heating pressures incurred during retort processing and cooling and which is able to withstand crush loads imposed by stacking.